(a) Field of the Invention
The invention relates to a new type of reflecting mirror that is made of a fluid, such as a ferromagnetic fluid, that responds to forces induced by magnetic fields, so that one can shape the reflecting surface as desired with an appropriately shaped magnetic field. To modify the reflective properties of the optical element surface, a reflective layer may be deposited on the magnetically sensitive fluid.
(b) Description of Prior Art
Optics is an enabling technology and optical elements are used in a large variety of technological applications. In general, it can be stated that the optical design of any instrument can be simplified by using aspheric surfaces. Aspheric surfaces, with the notable exception of a paraboloid, are difficult to obtain with solid optics. There are thus a vast number of possible applications for magnetically deformable liquid mirrors. For example, the surface of a liquid mirror deformed by magnetic forces could be segmented into multiple micromirrors that could individually be shaped at will.
Several fluids are affected by magnetic forces. For example, it is well known that the introduction of ferromagnetic particles in water or oil renders the fluid ferromagnetic. It is also known that currents sent through a conducting fluid generate magnetic fields so that the fluid can respond to the force resulting from an externally applied magnetic field.
Flat as well as parabolic rotating liquid mirrors have previously been made using liquid metals such as mercury and gallium. These are monolithic mirrors consisting of a liquid metal that is poured over into a solid container. However, they cannot be shaped by magnetic fields. Liquid mirrors using colloidal particles have been proposed (E. F. Borra, A. M Ritcey, E. Artigau, 1999 “Floating Mirrors”; Astrophysical Journal Letters, 516, L115). The mirrors discussed by Borra et al, are either flat or can be shaped by rotation. Magnetically shaped liquid mirrors have been described in the scientific literature. These mirrors are monolithic mirrors consisting of liquid mercury in which one may, or may not, introduce colloidal particles therein to render it ferromagnetic, after which they are poured into a solid container. Whitehead and Shutter (Astrophysical Journal Letters 1994, Vol. 418, pp. L139-L141, and U.S. Pat. No. 5,650,880) propose to use ferromagnetic liquid mercury specifically to transform the parabolic shape resulting from a rotating liquid into a sphere. Ragazzoni, R.; Marchetti, E. “A Liquid Adaptive Mirror”; Astronomy and Astrophysics vol 283, pp. L17-L19 propose to use monolithic liquid mercury driven by an electrical current flowing in the liquid and magnetic coils, specifically to be used as a deformable surface, in instrumentation used by astronomical telescopes, that compensates for wavefront distortion introduced by the earth atmosphere. They also made and tested a prototype. Ragazzoni, R., Marchetti, B., Claudi, R. “Magnetic Driven Liquid Mirrors in Orbiting Telescopes”; Astronomy and Astrophysics Supplement, v.115, p.175 propose to use ferromagnetic liquid mercury as the primary mirror of a space-based astronomical telescope. Mirrors using magnetic forces to deform a solid reflective membrane have been demonstrated by Cugat, et al, “Deformable Magnetic Mirror for Adaptive Optics: Technological Aspects”; Sensors and Actuators A, 89 (2001). Their approach uses permanent magnets glued or otherwise attached to the underside of a membrane. The magnets are then pulled by an array of coils separated from the membrane by an air gap. All these mirrors are high-reflectivity mirrors.
However, ferromagnetic mercury mirrors are massive due to the density of mercury. For the same magnetic field, larger deformations can be obtained with lower density materials. Moreover, mercury is toxic and it is a handicap for several applications. Finally, only high-reflectivity magnetically shaped mirrors have been developed; for applications such as ophthalmology, low-reflectivity mirrors are suitable. There therefore exists a need for new liquid mirrors shaped with magnetic fields.